When a profile of a thin-plate-like semiconductor wafer (an example of a workpiece, hereinafter referred to as wafer) is measured, a non-contact profile measuring apparatus using an interferometer is being widely used. The apparatus receives an interfering light containing a measurement light that is a reflected light when one of two branched light beams is reflected by a surface of a workpiece and a reference light that is a reflected light when the other light beam is reflected by a predetermined reference surface, and obtains a surface profile (surface height distribution) of the workpiece from an interference image that is formed based on the interfering light. Hence, since the surface profile of the wafer can be measured in a non-contact manner, the surface profile can be measured without a scratch or the like being generated on the surface of the wafer unlike when the profile is measured with a profile measuring instrument with a stylus. When the profile of the wafer is measured, the profile of the entire surface has to be measured. Hence, the measurement is generally performed while a circumferential edge portion of the wafer is supported (generally by three-point support).
Meanwhile, if the thin-plate-like workpiece (for example, with a thickness smaller than 1 mm) such as the wafer is supported only at the edge portion, the wafer vibrates with a slight wind pressure or by vibration or the like of other machines. The vibration has a non-negligible amplitude in the profile measurement for the wafer because the profile measurement needs extremely high measurement accuracy (for example, with an error of 20 nm or smaller). To prevent such vibration of the wafer, PTL 1 describes a method of restricting a wafer from vibrating by arranging a transparent rigid body near the wafer. However, this method has a problem such that an interfering light may be disordered because the transparent rigid body is inserted into an optical path. Also, PTL 2 describes a profile measuring apparatus that branches two-type measurement lights with slightly different frequencies into two lights, guides the lights to heterodyne interferometers at front and back surfaces of a workpiece, and measures a thickness of the workpiece by reversing the relationship between an object light and a reference light by the front and back heterodyne interferometers. With the technique described in PTL 2, by obtaining a difference between detection signals of the front and back heterodyne interferometers, the effect of displacement of the workpiece due to the vibration is eliminated, and the thickness can be measured with high accuracy without being affected by the vibration of the workpiece. Further, PTL 2 describes that the branched lights of the two-type measurement lights immediately before the lights are incident on the front and back heterodyne interferometers interfere with each other, and an intensity signal of the interfering light is used as a reference signal for the detection signals of the heterodyne interferometers. Accordingly, a measurement error resulted from fluctuation in phases of the two-type measurement lights generated in optical paths from a light source toward the two heterodyne interferometers can be eliminated.
However, even with the technique described in PTL 2, if the phases of the two-type measurement lights in the optical paths from the light source to the two heterodyne interferometers fluctuate at high speed, a circuit for detecting the phases cannot properly follow the speed of change. For example, if the two-type measurement lights are transmitted from the light source to the two heterodyne interferometers through optical fibers, the optical fibers may vibrate at high speed depending on the surrounding environment, and the phases of the two-type measurement lights may fluctuate at high speed. Then, with the technique described in PTL 2, processing for eliminating the fluctuation in the phases of the two-type measurement lights by using the reference signal does not properly function. Hence, even with the technique descried in PTL 2, the measurement error resulted from the fluctuation in the phases of the two-type measurement lights may not be reliably eliminated.